642 Types of Steam Traps and Their Selection

Various types of steam traps are available on the market, and the selection of the best trap for a given application is an important one. Many manufacturers produce several types of traps for specific applications, and manufacturers' representatives should be consulted in arriving at a choice. This section will give a brief introduction to the subject and comment on its relevance to improved energy utilization in steam systems.

Steam traps may be generally classified into three groups: mechanical traps, which work on the basis of the density difference between condensate and steam or air; thermostatic, which use the difference in temperature between steam, which stays close to its saturation temperature, and condensate, which cools rapidly; and thermodynamic, which functions on the difference in flow properties between liquids and vapors.

Figures 6.10 and 6.11 show two types of mechanical traps in common use for industrial applications. Figure 6.10 illustrates the principle of the "bucket trap. " In the trap illustrated, an inverted bucket is placed over the inlet line, inside an external chamber. The bucket is attached to a lever arm which opens and closes a valve as the bucket rises and falls in the chamber. As long as condensate flows through the system, the bucket has a negative buoyancy, since liquid is present both inside and outside the bucket. The valve is open and condensate is allowed to drain continuously to the return line. As steam enters the trap it fills the bucket, displacing condensate, and the bucket rises, closing off the valve. Noncondensable gases, such as air and CO2, bubble through a small vent hole and collect at the top of the trap, to be swept out with flash steam the next time the valve opens. Steam may also leak through the vent, but it is condensed on contact with the cool chamber walls, and collects as condensate in the chamber. The vent hole is quite small, so the rate of steam loss through this leakage action is not excessive. As condensate again begins to enter the bucket, it loses buoyancy and begins to drop until the valve opens and again discharges condensate and trapped air.

0 0

Post a comment